This invention relates generally to filter network and more specifically to a switched capacitor high pass filter in which the instantaneous coupling loop between input and output is interrupted.
The need to miniaturize filters has led designers to search for filter techniques compatable with integrated circuits. Initially, monolithic operational amplifiers were used in resistor-capacitor (RC) feedback networks which provided good performance without substantial size reduction. These networks have resisted inclusion into monolithic form due to the difficulty in achieving precise RC values in the monolitic process.
For filters in the audio and supra-audio frequencies, where long time constants in small semiconductor areas are required, sampled data techniques employing MOS transistors have become useful. Switched capacitor sampled data circuits utilize the fact that when a capacitor is switched between a signal to be sampled and a voltage amplifier at a frequency many times the frequency of the sampled signal it will simulate the circuit behavior of a resistor. The switched capacitor, in a proper circuit arrangement, can also be used as an integrator. Workers in the field have implemented sampled-data versions of second order (biquad) active filters using switched capacitors to simulate resistors. However, higher order filters realized by cascading these second order sections exhibit a sensitivity to component variation which preclude their use in high-precision filter applications.
High order precision filters have been achieved using "active ladder" or "leap frog" sampled data filters which have very low sensitivity to component variation in relation to cascade filters when the clock frequency is high. The low sensitivity of these filters is also nearly independent of sampling frequency, thereby permitting the use of clock frequencies many times higher than the passband frequency and reducing the requirements of the antialiasing prefilter. These filters become more sensitive at lower clock frequencies when the switched capacitor approximations exhibit large phase errors in the integrators. In addition, this type of filter can be organized so that there is a close correspondence between it and passive inductor-capacitor (LC) ladder networks. By exploiting this correspondence, the extensive tables and programs available for LC networks can be used to considerably reduce design effort required to achieve a given filter.
Active ladder filters have certain non-idealities which can affect the performance of switched ladder filters. Among the most significant are the amplifier DC offset voltage accumulation, finite integrating amplifier open loop gain, capacitor ratio errors and parasitics, noise, amplifier frequency response, and instabilities of certain filter configurations. One of these filters configurations is the higher order high pass filter which, due to the finite gain of the amplifiers combined with the near unity gain of the feedback loop, is subject to instability or long settling times. Others have used impedance scaling or element value changes to overcome the stability problem but an easily modeled and implemented solution has not, until now, been achieved.